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Kevlars 10-26-2011 11:39 PM

MDS Question
Ok .. I need some help in understanding how MDS works in the Hemi. When 4 Cylinders turn off I assume that the pistions still go up and down.. what happens to the spark from the plugs do they spark or not? --- so my question is this ... are we going to see more wear on 4 of the cylinders and their parts while the other 4 have less wear? or do they rotate on which cylinders are in operation?


phisch 10-27-2011 06:09 AM

Re: MDS Question
i believe the computer alternates which cylinders are deactivated. there should be equal wear across the motor. maybe even less wear compared to a normal v8 if you do a lot of highway driving because of this.

sined240 10-27-2011 06:26 AM

Re: MDS Question
I believe it deactivates the injectors and maybe the spark as well. It doesn't make sense to inject fuel and not use it. Whole point of fuel economy...

andyh 10-27-2011 07:48 AM

Re: MDS Question

Originally Posted by phisch (Post 504770)
i believe the computer alternates which cylinders are deactivated.

The same 4 cylinders are deactivated. No alternation.

Celly 10-27-2011 08:00 AM

Re: MDS Question
The same 4 cyl's get deactivated each time.

What happens is that when MDS activates, is an initial Air-Fuel charge is injected, it is ignited, but the exhaust ports don't open. This charge is left in the cylinder and acts as a spring as it is constantly being compressed and then assists in pushing the deactivated cyl down on it's normal power stroke. This is also said to help keep heat in the cyl while it is deactive.

This 'Spring' effect also helps the engine run smoother when it is in 4cyl mode, since it helps even out dead spots in the power strokes.

When MDS disables, the Exhaust port opens releasing the kept exhaust charge, and then the intake vales open giving the cylinder a fresh air/fuel charge and it moves on it's way.

As far as wear across the Cyls. It's hard to say. You might see less fouling on the spark plugs over time, or possibly less deposits on the cyls. But, it'll be minimum since the engine is not in a 'Stress' situation when MDS fires. It only runs in absolute optimal situations -- ie, when there is little to no stress on the rings and pistons.

MDS is a brilliant technology. It was very well developed and thought out. I recommend Spending some time on the Google if you want to learn more.

phisch 10-28-2011 01:52 PM

Re: MDS Question
apparently my dealer was incorrect. that never happens, does it?

Uncle Paul 10-28-2011 02:39 PM

Re: MDS Question
Hemi with cyl deactivation recommends plug change (16) at 36,000 miles.

Kevlars 10-29-2011 11:26 AM

Re: MDS Question
It would be been a lot better if the comptuer alternated ... oh well.

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